Temperature compensated resonant cavity



Jan. 22, 1957 c. H. BREDALL ETAL 2,779,004

TEMPERATURE COMPENSATED RESCNANT CAVITY Filed Feb. 4, 1955 1N? 'ENTORS CHAR LES H. BREDALL JOHN B. SH ANNON United States Patent O TEMPERATURE COMPENSATED RESONANT CAVITY Charles H. Bredall, Pacific Palisades, and John B. Shannon, Camarillo, Calif.

Application February 4, 1955, Serial No. 486,302

6 Claims. c1. 333-82) (Granted under Title 35, U. s. Code (1952 sec. 266) cables. The connector comprises a body member having a transverse opening conforming to and slightly smaller than the cross-sectional configuration of the cavity to insure a force fit and a snug electrical connection without resort to brazing or the like. A longitudinal passage in the body intersecting the transverse opening enables the introduction of an inner coaxial conductor or probe into the cavity with clamping means provided for electrically and mechanically connecting the outer coaxial conductor to the body member.

A principal object of this invention is to provide a reactive device with a rigid dielectric material having a dielectric constant capable of stabilizing the reactance varitions due to dimensional changes in the device with which will enable the degree of coupling to be adjusted.

It is well known that the resonant frequency of a I resonant cavity fluctuates with dimensional changes caused by temperature variations. One known method employed to reduce this effect on frequency is to construct the cavity of an iron-nickel alloy, known as Invar, which has a very small coefficient of thermal expansion. As would be expected, the cost of resonant cavities made of this material is quite expensive and, further, cavities of this type do not provide complete compensation especially where the cavity is subjected to extreme temperature variations, as may occur in aircraft installations.

Another temperature compensating means that has been utilized consists of filling a resonant cavity made of conventional material with a gas having a dielectric constant which at high frequencies decreases with increasing temperature. Although use of such a gas obviates the disadvantages previously mentioned, there are inherent disadvantages in the use of a gaseous dielectric material, namely, a gas-tight cavity construction is required to prevent the gas from escaping which increases construction costs, and also reduces the versatility of the cavity so far as adjusting the degree of coupling.

Another disadvantage of both of the foregoing constructions resides in the necessity for providing additional means to support a coupling loop or probe extending within the cavity where it is subjected to severe shock and vibration.

According to this invention, the reactive device is provided with a solid dielectric substance having a dielectric constant that varies inversely with temperature and which substantially compensates for reactance variations due to dimtnsional changes in said device with changes in temperature. In a preferred embodiment the device is a resonant cavity, or in effect a short section of a transmission line, having inner and outer spaced conductors between which is positioned the solid dielectric material. The relative lengths of the dielectric material and the cavity depends on the relation between the coefficient of dielectric constant of the dielectric material and the coefficients of expansion of the cavity materials. The rigid dielectric material, which may be in a solid or granular form, is preferably located within the cavity at points of low impedance and also at the entrance position of a coupling probe to provide a support therefor when the cavity is subjected to shock or vibration.

Another feature of the invention resides in a novel connector for electrically coupling the resonant line or cavity to input and output conductors, such as R. F. coaxial Still another object is to provide a resonant cavity which is made of inexpensive materials and simple to manufacture.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is an elevation view of a cavity resonator;

Fig. 2 is a partial section of the resonator taken along line II-II of Fig. l; and

Fig. 3 is a top plan view of the resonator taken along line III--III of Fig. 1.

Referring now to the drawings where like reference characters designate like or corresponding parts throughout the several views, there is shown a resonant cavity 10 which may be used as a tuned filter or like component for radio or other electrical circuits. Resonant cavity 10 comprises an outer hollow conductor 12 and an inner conductor 13 securely disposed in spaced relationtherein forming in effect a short section of a transmission line. Outer conductor 12 can be a cylindrical or rectangular tube, or a drilled block and having conductive end closure walls 14 and 16 preferably sealed thereto. One end of inner conductor 13 is electrically connected to end closure 16 by a screw 18 and extends toward end closure 14 but is spaced therefrom at' 20 forming a /1 wavelength resonator. Space 20 may be varied by an adjustable screw 22 threadedly mounted in end closure 14 and locked by a nut 23, screw 22 being movable along the longitudinal axis of inner conductor 13 cooperating with the end of inner conductor 13 as a variable capacitor for tuning the resonance of the cavity to the input frequency.

Inner and outer conductors 13 and 12, which may be constructed of copper and brass, respectively, expand or contact with changes in temperature, and when the outer and inner conductors are made of such metals having similar temperature coefficients of expansion, spacing 20 remains substantially constant throughout any tempera compensate for resonant frequency variations due to the dimensional changes in the resonator with changes in temperature. The length of the dielectric. material .to provide such a compensation factor will depend on the temperature coefiicient of dielectric constant of the dielectric material selected. 7 Where thedielectric materials fill less than the entirecavity that which remainstcan be an air space] One dielectric material having the foregoing properties that has been found'towork satisfactorily is polyethylene, the dielectric material present ina type RG-l7/U radio frequency cable manufactured by the Federal Telephone and Radio Corporation, and the length of such dielectric material necessary to accomplish the desired results being determined by the following derivation:

The physical length of the resonant lineis: 1 l=la+lp where i l=length of the inner conductor l=length of'the airspace between the free end of the inner conductor and polyethylene material and l =lengthof the polyethylene material.

The 'known constants arei Cc=temperature coeflicient of expansion of copper and brass=0.18 1O /-C; (approx) C =temperature coefficient of expansion of polyethylene=0.l8 l0- /C. (approx.) Ck=temperature coefficient of dielectric constant of polyethylene=-2 10- /C. (approx;)-- K=dielectric constant of polyethylene=2.25 (approx.).

At temp T electrical length of resonant line is:

At temperature T', such that T-T=AT, the electrical length is: Y

where, l=l(l|-CAT) rr=z 1+ m /K= /K(1+ Ck'AT) v with the proper choice of l and l complete compensation will rcsultmaking the electrical lengths at Land L equal; Assuming such compensation is' achieved, it is desired to findthenecessary relation between 'l andl Thus, as-j' sume:

Substituting (2) and (3) into (4) Dividing all terms by lpCcAT and collecting terms:

'QQA Fi Ki& t I a 1+CQT) f W1. C'GATH .1 Values of AT are restricted 'to temperature variations over which the variation of dielectric constant with temperature is essentially linear, thus C1; is considered' constant over the temperature range normally encountered. For simplicity assume AT=1 C. and evaluate (5) with" the constants noted above: i

Any correction of this ratio to allow for electrical discontinuity at the junction of the air and polyethylene dielectric is negligible. Using polyethylene at one-third, the length of the inner conductor1 3 exhibited aninconsequential? change of resonant frequency with temperatures frorn-25 C. to C.

The term solid dielectric material is meant to include a dense and rigid material, such as polyethylene, or any confined granular material having the desired negative temperature coeificient of dielectricconstant, as long as the dielectric material is-capable of; supporting against shock and vibration either a magnetic loop or electrostatic coupling probe" that may be inserted into the cavity.

Referring again to Fig. 2, another novel feature of the invention resides in providing an electrical connector 26 for coupling suitable coaxial cablesto resonantcavity 1'0 to'provide an inductive or capacitative coupling, the latter being illustrated.

The coupling or connector comprises a body member 28, which preferably isa cylindrical rod-having formed therethrough a transverse opening 3d conforming to but slightly smaller than the configuration of outer conductor 12 to provide a force .or press fit andconsequently a good electrical connection without the need of soldering, brazing or othertime-consuming means of making the connection. Whena cylindrical cavity is employed, opening 30 may be drilled slightly smaller in diameter than both outer conductors 12 and the diameter of rod body member'28, the latter being such that a-thin and reduced wall section 32 remains on each side of the body member giving the body member sufiicient resilience to grip outer conductor 12 at all times, which is especially important if the cavity is to be subjected to wide temperature variations." In the intersectionof two cylinders as shown in Fig.2, wall sections'32 are smaller in height atthe thinnest point forming a recessed mouth to; opening 30 at each side of the body member which greatly facilitates the insertion of outer conductor 12 into the body during assemblyby'enabling the former to be gradually forced into the opening.

Body'memb'er 28 also has a drilled longitudinal passage 34 intersecting opening 30 and through which may be insertedat each end an inner conductor 36 and insulation 37Qof input and output R. F. coaxial cables 38 to be electrically coupled to the cavity. The details ofonly one ofthe coaxial cables 38 and the corresponding section of connector body 28 is illustrated sinccthe othercable'and body section can be of a duplicate construction. The wall of outer conductor 12 and dielectric material24, when it consists of a rigid material such as. polyethylene,

are provided with a pair of drilled passages 40 andAZ,"

respectively, aligned with passages 34.:1to;permit the .free

endf oi inhe'r coaxial c onduetor s 36,, which function-as about 0.10, inchthick .a convenient range of adjustability is afforded by'use of as many discs as are needed. 'The peripheral surface of inner conductor 13 may be flattened f at 46Ton each .side to receive thedielectric discs'44and ensure a constant coupling. The end ,of outer coaxial conductor 48, usually of a braided construction, iscXpanded andclamp'ed 'by a washer 50 and bolts 52 to the end face 49 ,QfJCOIlIpQCtQY body. 2$ i0; electrical connection to. outerconductor 12., WasherQ SO has a bore opening 54 through whic'lrhais;

passed the coaxial cable prior to assembly. Connector body face 49 is formed with an integral collar 56 which cooperates with a beveled neck 58 of Washer 50 to wedge outer coaxial conductor 48 and coaxial cable jacket 60 against the connector body and ensure a sealed joint. This clamping action also forces the inner coaxial conductor 36 inwardly which seats discs 44 against inner conductor 13.

The use of a solid dielectric material in a transmission line which may form a part of a cavity resonator furnishes the needed temperature compensation for physical variations without resort to expensive conductive metals, such as Invar or to complicated constructions. The solid dielectric material, which can be in a rigid or loose form, functions also to support the magnetic loop or electrostatic probe within the transmission line against shock and vibration. When a compact dielectric material is employed, such as polyethylene, the dielectric material may be fabricated of a single piece, as illustrated, or separated into spaced beads secured within the outer conductor of the transmission preferably located at points of low impedance and at the entrance area of the coupling probe into the transmission line to provide the needed support therefor. A simple force-fit connector is provided for coupling the transmission line to R. F. coaxial cables which connector eliminates costly and tedious soldering operations or the like. The inner and outer conductors of R. F. cables are securely attached to the transmission line by the connector and sealed from at mospheric elements.

Obviously many modifications and variations of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

We claim:

1. A temperature-compensated electrical apparatus including an encolsed resonant device adapted to receive a coupling conductor extending therein, a solid dielectric substance in said device and capable of supporting said coupling conductor, said dielectric having a dielectric constant that varies inversely with temperature substantially to compensate for resonant frequency variations due to dimensional changes in said device with changes in temperature.

2. A temperature-compensated electrical apparatus including an enclosed resonant device adapted to receive a coupling conductor extending therein, a solid and rigid dielectric substance in said device and having an aperture to receive and support said coupling conductor, said dielectric having a dielectric constant that varies inversely with temperature substantially to compensate for resonant frequency variations due to dimensional changes in said device with changes in temperature.

3. A temperature-compensated resonant cavity comprising an elongated outer hollow electrical conductor made of brass and a concentrically spaced inner electrical conductor made of copper secured to one end of said outer conductor and extending toward and spaced from the other end of the outer conductor, a solid dielectric material positioned in the space between said inner and outer conductors and having a dielectric constant that varies inversely with temperature substantially to compensate for resonant frequency variations due to dimensional changes in said resonant cavity with temperature changes, said dielectric material being polyethylene and having a length substantially one-third of the length of the inner conductor.

4. A temperature-compensated resonant cavity comprising an elongated outer hollow-electrical conductor and an inner spaced electrical conductor secured to one end of said outer conductor and extending toward and spaced from the other end of the outer conductor, a solid dielectric body positioned in the space between said inner and outer conductor, said dielectric having a dielectric constant that varies inversely with temperature substantially to compensate for resonant frequency variations due to dimensional changes in said resonant cavity with changes in temperature, a coupling body for connecting input and output coaxial cables to the cavity, said dielectric body, outer conductor, and coupling body each having aligned openings for receiving the inner coaxial conductor.

5. A temperature-compensated resonant cavity comprising an elongated outer hollow electrical conductor and an inner spaced electrical conductor secured to one end of said outer conductor and extending toward and spaced from the other end of the outer conductor, a solid dielectric body positioned in the space between said inner and outer conductors, said dielectric having a dielectric constant that varies inversely with temperature substantially to compensate for resonant frequency variations due to dimensional changes in said resonant cavity with changes in temperature, a coupling body for connecting input and output coaxial cables to the cavity, said dielectric body, outer conductor, and coupling body each having aligned openings for receiving the inner coaxial conductor, an insulating disc positioned in the opening of the dielectric body for spacing the inner cavity conduct-or from the inner cable conductor.

6. A temperature-compensated resonant cavity comprising an outer cylindrical hollow electrical conductor and a concentrically spaced inner electrical conductor secured to one end of said outer conductor and extending toward and spaced from the other end of the outer conductor, a solid dielectric body positioned in the space between said inner and outer conductors, said dielectric having a dielectric constant that varies inversely with temperature substantially to compensate for resonant frequency variations due to dimensional changes in said resonant cavity with changes in temperature, a coupling for connecting input and output R. F. coaxial cables to the cavity, said coupling including a coupling body formed of a round rod member having a transverse circular opening with a smaller diameter than the diameter of the outer cylindrical conductor to receive the latter in a force fit, said dielectric body, outer cylindrical body, and coupling body each having aligned openings for receiving the inner coaxial conductor, and a washer mounted on the coupling body for clamping the outer coaxial conductor to the coupling body.

References Cited in the file of this patent UNITED STATES PATENTS 

